Micro-cavity laser

ABSTRACT

The present invention is a micro-cavity laser and methods related thereto. In the preferred embodiments, the micro-cavity laser comprises a laser pump signal in a fiber waveguide which is optically coupled to a micro-cavity resonator through a fiber taper. The micro-resonator includes a gain medium necessary for lasing action. The lasing frequency can be determined based upon the gain medium, the micro-cavity structure, as well as frequency selective elements such as gratings incorporated into the micro-cavity. The tapered fiber waveguide permits the micro-cavity laser to operate without a break in the fiber waveguide. In the preferred embodiments, the micro-cavity resonator is constructed from a doped silica or a semiconductor material. The present invention provides a compact laser with improved emissions and coupling efficiencies. Alternative configurations include multiple micro-cavities on a single fiber waveguide and/or utilizing multiple waveguides attached to one or more micro-cavity resonators. The laser can be made to operate in a continuous-wave as opposed to self-pulsing mode.

[0001] This application claims priority on U.S. provisional applicationNo. 60/188,325, filed Mar. 9, 2000, and entitled, “Fiber-CoupledMicrosphere Laser.” The disclosure of the foregoing is incorporated byreference herein as if set forth in full hereat.

FIELD OF INVENTION

[0002] The field of the invention relates to lasers and certain relatedmethods, and in particular to micro-cavity lasers and related methods.

BACKGROUND OF THE INVENTION

[0003] In the now rapidly expanding technology relating to the use ofoptical waveguides and in particular fiber optic waveguides, a number ofdiscrete devices and subsystems have been developed to modulate, routeor otherwise control, optical beams that are at specific wavelengths.Present day communication systems increasingly use individual waveguidesto carry densely wavelength multiplexed optical beams. Thus, there is aneed for a self-contained device and related methods which can induce alased output in a frequency range of interest. Currently, thetelecommunications industry uses frequencies in the 1550 nm range.

[0004] It is known to one of ordinary skill in the art how to couple awaveguide to an optical resonator so as to transfer optical power to theresonator from the waveguide or from the waveguide to the resonator. Itis also known to one of ordinary skill in the art that power circulatesin a resonator preferentially at resonant frequencies corresponding tooptical modes of the resonator. For the purposes of discussion the termsresonance and optical mode will be used interchangeably herein. Likewisethe principles associated with lasing action in resonators and inparticular rare earth doped resonators and micro-resonators are wellunderstood to one of ordinary skill in the art. The terms micro-cavity,resonator, micro-resonator will be used interchangeably herein.Discussion of these concepts can be found in one or more of thefollowing references, the disclosure of each of which is incorporated byreference herein as if set forth in full hereat: V. Lefevre-Seguin andS. Haroche, Mater. Sci. Eng. B48, 53 (1997); J. C. Knight, G. Cheung, F.Jacques, and T. A. Birks, Opt. Lett. 22, 1129 (1997); M. Cai, O.Painter, and K. Vahala, Phys. Rev. Lett. 85,74 (2000); M. Cai and K.Vahala, Opt. Lett. 25, 260 (2000); V. Sandoghdar, F. Treussart, J. Hare,V. Lefevre-Seguin, J. M. Raimond, and S. Haroche, Phys. Rev. A 54, 1777(1996); W. von Klitzing, E. Jahier, R. Long, F. Lissillour, V.Lefevre-Serguin, J. Hare, J. M. Raimond, and S. Haroche, Electron. Lett.35, 1745 (1999); P. Laporta, S. Taccheo, S. Longhi, O. Svelto, and C.Svelto, Opt. Mater. 11,269 (1999); V. B. Braginsky, M. L. Gorodetsky,and V. S. Ilchenko, Phys. Lett. A 137,393 (1989); A. Serpenguzel, S.Arnold, and G. Griffel, Opt. Lett.20, 654 (1995); V. S. Ilchenko, X. S.Yao, and L. Maleki, Opt. Lett. 24,723 (1999); M. L. Gorodetsky and V. S.Ilchenko, J. Opt. Soc. Am.B 16, 147 (1999); T. Baer, Opt. Lett. 12, 392(1987); G. H. B. Thompson, Physics of Semiconductor Laser Devices(Wiley, New York, 1980); T. Mukaiyama, K. Takeda, H. Miyazaki, Y. Jimba,and M. Kuwata-Gonokami, Phys. Rev. Lett. 82, 4623 (1999);

[0005] The theoretical concept of inducing lasing action in amicro-resonator doped with Nd is discussed by F. Treussart, et al., inEur. Phys. J. D 1, 235 (1998), the disclosure of which is incorporatedby reference herein as if set forth in full hereat. This reference,however, presents a device which relies on the use of prisms to coupleto the laser resonator. Such a configuration presents many difficultiesand limitations on its use in the field, as it requires delicate andprecise alignment, is bulky and not easily adaptable common use and doesnot produce an output frequency which is currently of most use in thetelecommuting industry. Additional limitations of these and otherdevices include low emission and coupling efficiencies.

[0006] The present invention overcomes these and the other limitationsof the prior art by providing a compact, self-containable laser sourcethat is directly coupled to an optical fiber waveguide. Optical fibers,in addition to being very important in modem optical communicationssystems, provide a very convenient means to convey both optical pumppower to the laser as well to convey emitted laser radiation from thelaser resonator. The ability to directly couple laser emission to anoptical fiber is therefore of great practical significance. The outputfrequency of the present invention can be tuned both by design (based onchoice of certain materials and/or dopants utilized) and dynamically (byvarying the frequency of the laser pump signal) and by incorporation ofgrating structures into the micro-cavity. The present invention alsoprovides a laser source with improved emissions and increased couplingefficiency between the waveguide and the resonator. Finally, the each ofthe preferred embodiments can be made to be robust and easy to implementin a variety of configurations and uses.

SUMMARY OF THE INVENTION

[0007] The present invention is directed to a micro-cavity laser andcertain related methods. The devices and methods of the presentinvention are useful for creating laser signals having a frequencywithin a desired range by optically coupling an optical pump signal in awaveguide to a micro-cavity optical resonator, which resonator includesan active medium which is capable of providing optical gain upon pumpexcitation and which resonator and pumped active medium result in lasingaction at a frequency within the desired output range. In the preferredembodiments, the waveguide is a fiber waveguide of any configuration andthe coupling between the fiber waveguide and the resonator is by meansof an optical couple between a fiber taper in the fiber waveguide andthe micro-cavity optical resonator. In the preferred embodiment thefiber waveguide serves to both transport optical pump power to theresonator to excite the amplifying medium as well as to collect lasingemission from the laser cavity and transport it to elsewhere. The fiberwaveguide and the resonator are preferably critically coupled at thepump wavelength so as to maximize pump power coupling to the activemedium. In addition, it is possible and important to phase match thefiber taper and the micro-cavity resonator so as to maximize thecoupling efficiency between these two elements of the present invention.

[0008] In another embodiment two fiber waveguides are coupled to themicro-cavity and each is optimized for coupling of pump power orcollection of laser emission. In this embodiment phase matching could beemployed to perform this optimization.

[0009] The micro-cavity optical resonator can have a variety of shapesincluding, without limitation, a microsphere, one or more micro-rings,racetracks or disks incorporated on a substrate or one or moremicro-rings or disks formed on the fiber waveguide itself. Indeed, it ispreferable in certain applications for there to be more than onemicro-cavity resonator on a single fiber waveguide, for example increating a multi-wavelength laser array along the fiber waveguide.

[0010] The output of the micro-cavity laser of the present invention canbe tuned by varying the pump wavelength and/or utilizing differentmaterial composition for the micro-cavity optical resonator. Inaddition, internal structures such as optical gratings can be added tothe optical path within the resonator so as preferentially select aparticular optical mode for lasing and in turn the frequency. The lasercan also be made to operate continuous wave or self-pulsing.

[0011] Accordingly, it is an object of the present invention to providea micro-cavity laser having the advantages detailed herein.

[0012] This and other objects of the invention will become apparent tothose skilled in the art from a review of the materials containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The accompanying drawings, which are incorporated in, andconstitute a part of the Specification, illustrate presently knownpreferred embodiments of the present invention, and together with theproceeding general description and the following Detailed Description,explain the principles of the invention.

[0014] In the drawings:

[0015]FIG. 1 is an illustration of a micro-cavity laser of the presentinvention;

[0016]FIG. 2 is a plan view illustration of a fiber taper and amicro-cavity resonator;

[0017]FIG. 3 is an image of a fiber taper in contact with the equator ofa microsphere resonator;

[0018]FIG. 4 is an illustration of a fiber taper coupled with theequator of a microsphere microcavity resonator;

[0019]FIG. 5 is a graph illustrating the phase matching of a fiber taperfundamental mode and a microsphere resonator fundamental modes;

[0020]FIG. 6 is an image of the green-up converted photo-luminescencefrom a fiber taper-pumped microsphere, where the pump wavelength istuned close to a fundamental whispering gallery mode;

[0021]FIG. 7 is an image of photo-luminescence spectra [taken at point(a) in FIG. 8, below] of a microsphere resonator for an annular pumpregion about the equator. The photo-luminescence (inset) is taken atpoint (b) in FIG. 8 (with a wavelength range matching that of the mainspectra), where the side-mode suppression is 26 dB;

[0022]FIG. 8 is a spectral output of collected laser output power versusabsorbed pump power in the microsphere (L_(out)−L_(in)). Inset, spectraloutput of a Fabry-Perot filter, showing the single-mode nature of themicro-cavity laser of the present invention, where for reference asingle-frequency laser with a known line-width of 300 kHz is also shown;and

[0023]FIG. 9 shows (a) a single sphere system, and (b) a bi-spheresystem in which two spheres have been placed on the same taper andpumped by a single 980 nm laser source, producing two separate laserlines, at 1533 and 1535 nm.

DETAILED DESCRIPTION OF THE INVENTION

[0024] Referring hereafter to the figures generally, and in particularto FIGS. 1-2 here, the present invention is a compact and highlyefficient laser 2. In its preferred embodiment, the present inventionutilizes transmission media 4; high-Q micro-cavity optical resonators 6;active media associated with the optical resonators to facilitate thelasing of a signal within a frequency band of interest; and, opticalpumps to excite the active media. As described below and as will beunderstood by those skilled in the art, numerous additionalimplementations of this structure and/or method can be made withoutdeparting from the scope or spirit of the invention as described herein.

[0025] The transmission media 4 is preferably a fiber waveguide 5 of anytype. This includes, without limitation, cylindrical, elliptical,etched, “D”-shape and “panda” fiber configurations as well as polishedfiber half-blocks. In the preferred embodiment, a fiber taper 12 isprovided in the fiber waveguide 5 between a first and second end of thefiber waveguide 5 as is best illustrated in FIG. 2. The taperedsections, 15, 16 and intermediate waist region 14 of the waveguide maybe provided, as is known, by stretching the waveguide under controllabletension as it is softened by one or more fixed or movable heat sources(e.g., torches). Commercially available machines can be used for thispurpose in production environments. The consequent reduction in diameterof about one or more orders of magnitude reduces the central core in thecore/cladding structure of the optical fiber to vestigial size andfunction, such that the core no longer serves to propagate the majorityof the wave energy. Instead, without significant loss, the wave power inthe full diameter fiber transitions into the waist region, where poweris confined both within the attenuated cladding material and within afield emanating into the surrounding environment. After propagatingthrough the waist region 14, exterior wave power is recaptured in thediverging tapered region 16 and is again propagated with low loss withinthe outgoing fiber section 18, as illustrated in FIGS. 1 and 2.

[0026] The high Q resonator 6 in this example is coupled to theexternally guided power about the waist region 14 of the waveguide. Thatis, at all times there is a coupling interaction from the principalfiber into the interior of the resonator 6 via the resonator periphery.The resonator 6 additively recirculates the energy with low loss in thewhispering gallery mode (“WGM” or WG mode”), returning a part of thepower to the waveguide at the waist 14. When a resonance exists at thechosen wavelength, the resonator 6 functions with effectively totalinternal reflection and with minimal internal attenuation and radiativelosses. However, the emanating portion of the wave power is stillconfined and guided, so it is presented for coupling back into thewaveguide waist 14. Extremely high Q values (as much as 8 billion havebeen observed) exist in this whispering gallery mode. Different WGMdevices can be used for the present invention, including disks, rings,polygons, oblate and prolate spheroids. Furthermore, concentricity orapproximate concentricity may in some instances not be necessary, sincethe WGM effect can exist in non-concentric boundary structures such asellipses or race-track structures.

[0027] In the present invention, the resonator 6 is preferablyconstructed from a silica material. This provides the advantage of beingcompatible with many waveguide structures, most importantly,telecommunication fiber waveguides currently in use. Alternatively,resonators can be constructed in a semiconductor, utilizing any of theresonator configurations (e.g., disks, rings, polygons, oblate andprolate spheroids) discussed herein. Depending on the application inwhich the laser of the present invention might serve and/or the desiredfrequency bandwidth of the output, the material from which the resonatoris constructed may also include one or more additives (for example andwithout limitation, phosphate) intended to suppress undesirable higherorder modes and/or resonances in the resonator 6 at frequencies outsideof the desired output bandwidth.

[0028] In order for the micro-cavity resonator 6 to lase within adesired frequency bandwidth, an active media must also be present. Theactive media produces the optical gain necessary to permit lasing onceexcitation of the structure is initiated by one or more optical pumpsources. In the preferred embodiments, the present invention utilizesone or more dopants in the resonator 6 to serve as the active media. Thepreferred dopants include rare earth materials and particularly erbium,ytterbium, praseodymium, neodymium, holmnium, and thulium, either aloneor in combination with another dopant. The exact combination andconcentration of dopants depends on the wavelength band or bands soughtto be included in the output of the laser of the present invention.

[0029] The present invention also utilizes an alignment structure inorder to secure the position of the fiber waveguide 6 relative to themicro-cavity resonator 20. Many types of alignment structures are knownto those of ordinary skill in the art and may include, withoutlimitation, an etched substrate or the like. In addition, an alignmentstructure may include structures of the type disclosed in pending U.S.patent application Ser. No. ______, the disclosure of which isincorporated herein in full by reference. Illustrations of these andother embodiments are set forth in Vahala, et al., U.S. patentapplication entitled “Resonant Optical Filters”, Ser. No. ______, filedFeb. 16, 2001, the disclosure of which is incorporated herein byreference.

[0030] To induce a lasing action in the present invention, an excitationsignal must be provided to the resonator 6. In the first preferredembodiment, an optical pump 20 is provided to deliver the excitationsignal to the resonator 6. Alternative schemes of delivering anexcitation sources (e.g., and without limitation, by beam excitationincluding guided or unguided electrical and/or unguided light beams) canbe employed without departing from the scope of the present invention.

[0031] Without limiting the foregoing, in the first preferred embodimentan optical pump 20 is optically connected to a first end of the fiberwaveguide 5. The optical pump 20 transmits a signal along the waveguide5 and to the resonator 6 through the fiber taper 12 as discussed above.One or more excited laser signals in the resonator 6 are thencommunicated to the fiber waveguide 5 propagating both in the directionof the second end of the waveguide as illustrated in FIG. 5 (and towardsthe first end of the waveguide). In an alternative embodiment where theresonator is constructed from a semiconductor, the resonator 6 ispreferably pumped by an electrical excitation signal rather than anoptical signal, however, pumping in this configuration by a guided orunguided optical or alternative signal beam is also intended to beincluded within the scope of the present invention.

[0032] A significant advantage of the present invention over the work ofothers is the ability to couple directly to and from optical fiber.Important to this coupling is the ability to “phase match” the fibertaper 12 and the resonator structure 6 to maximize the couplingefficiency. This is done by proper selection of the diameter of thefiber taper 12 at the waist region 14. In so doing, it is possible tomatch the effective indexes of the fundamental taper mode and thefundamental mode of the resonator 6 (i.e., “phase matching”). Asillustrated in FIG. 5, where the resonator 6 is a microsphere 7, a 50micron diameter microsphere 7 phase matches a 1.38 micron diameter fibertaper 12. In the present invention, it is possible to demonstratecritical coupling with 26-dB on-resonance extinction and a matcheddual-taper add-drop filter with less than 0.5% scattering loss andnear-unity power transfer (on-resonance) between a fiber taper 12 and amicro-cavity resonator 6, where the resonator is a microsphere resonator7.

[0033] A laser of the present invention has been constructed and testedin the laboratory, and is described more fully below. It will beappreciated that this embodiment is but one of many embodiments of theinvention disclosed and claimed herein and is described as the currentlyknown best mode of the present invention rather than as a limitation ofthe invention itself.

[0034] In this embodiment and referring to all of the figures generally,a fiber taper 12 is placed in contact with Er:Yb-doped phosphate glassmicrosphere 9, to form a compact, low-threshold 1.5 mm wavelength fiberlaser source. A single fiber taper 12 is used to guide the pump 20 laserbeam to the surface of the microsphere 9, resonantly couple the pump 20into the sphere 9, and then collect the resulting laser emission. Theuse of a fiber taper 12 not only provides an efficient input and outputcoupling port but also plays an important role in producing single-modelasing. Finally, the fiber taper 12 forms a natural backbone forconnecting a series of different active and passive microcavity devices,with each device addressing a different wavelength signal. Theseadditional microcavity devices can be resonators, modulators, add/dropfilters, slicers, or any other device which can optically connected tothe fiber waveguide 5, preferably through the fiber taper 12 or one ormore additional fiber tapers on the fiber waveguide 5 so as to make suchconnections without breaking the fiber waveguide 5.

[0035] The microspheres used in this embodiment were formed fromphosphate glass heavily doped with Yb (20% by weight) and Er (0.5%).Kigre QX/Er phosphate glass has a transformation temperature of 450° Cand a refractive index of 1.521 at 1.5 μm. Absorption that is due to theF_(5/2)→F_(7/2) transition of the Yb³⁺ ions is strongly peaked around976 nm (±5 nm), with a value of ∝=4-5 cm⁻¹ (2×10³ dB/m). The F_(7/2)level of Yb³⁺ resonantly couples to the Er³⁺I_(11/2) level, which thenrelaxes to the I_(13/2) level. The 1.5-μm lasing transition is betweenthe ground-state I_(15/2) level and the I_(13/2) excited-state level ofEr³⁺, with a fully inverted gain per unit length exceeding 200 dB/m inthe 1500 nm band.

[0036] Fabrication of the microspheres and the fiber tapers is discussedin the references cited above and incorporated herein. In summary, asmall piece of the phosphate glass is melted in a crucible. With thephosphate still molten, the tip of a silica fiber taper, which has ahigher melting point, is placed into the melt. As the silica “stem” isextracted, a small phosphate taper is formed on the end of the silicataper. A CO₂ laser is used to melt the end of the phosphate taper,forming a spheroid under surface tension. The silica fiber stem isfinally placed in a fiber chuck and used as a handling rod to controland position the phosphate sphere. It is important to carefully controlthe temperature of these operations and to cool the sphere quickly in amanner which avoids crystallization of the phosphate in the spheroid toan extent which would interfere with the reflective properties of thespheroid as a micro-cavity optical resonator.

[0037] The fiber tapers for this embodiment were formed by takingstandard telecommunication 125 μm diameter silica fiber, heating a shortregion with a torch, and then slowly pulling the fiber ends to form anadiabatic taper region. In order to provide efficient coupling betweenthe fiber taper 12 and the microsphere, a fiber taper diameter must betailored for each different sphere size and WG mode of interest asdescribed above. Fine tuning of the coupling can further be performed bychanging the position of the sphere relative to the taper waist.

[0038] The resonant modes of nearly spherical dielectric particles canbe classified according to their polarization index p, radial modenumber n, and angular mode numbers l and m. Of special interest in thisembodiment are the WGM resonances, i.e., those with small radial modenumbers and large angular mode numbers. Excitation of WGMs within glassmicrospheres 7 via a fiber-taper 12 coupling has several distinctadvantages. Most important of these is direct coupling to and from theoptical fiber. In addition, alignment is built in, fabrication isrelatively simple, and as discussed above, index matching between thefiber taper 12 and the diameter of the WGMs of the microsphere 9 ispossible.

[0039] A magnified image of a coupled fiber taper microsphere is shownin FIG. 3. For the microsphere laser of the present embodiment, thediameter and eccentricity were determined by analysis of its resonantmode structure at 1.5 μm. The measured WG mode free-spectral range inl(FSR_(l)) for this microsphere is 1.1 THz (8.7 nm) at 1.5 μm, giving adiameter of 57 μm. The measured free-spectral range in m is 13 GHz for|m|≈l, with the resonant frequencies increasing with decreasing m value.This corresponds to a slightly oblate microsphere with an eccentricityof 2.4%. The pump wave in this embodiment is launched from a 980 nmwavelength, narrow-line width (<300-kHz), tunable external-cavity laserinto the fundamental mode of the fiber taper.

[0040] As discussed above, this embodiment also maximizes the efficiencyof the pumping of the microsphere 9 by providing a good match betweenthe fundamental mode of the fiber taper 12 and the WG modes of thesphere 9 and by matching the input coupling strength to the round-tripresonator loss (i.e., critical coupling). Owing to the large absorptionwithin the microsphere 9 at the pump band and the subsequent largeround-trip microsphere resonator loss, maximum power transfer isobtained for the fundamental WG modes (|m|=l), as the spatial overlapwith the fiber taper 12 is highest for the equatorial modes, resultingin higher input coupling strengths. For this sphere, a taper diameter of1.75 micrometers was used to phase match and selectively excite thelowest-order (n=1,2) fundamental WG modes of the sphere 9.

[0041] The pump volume within the micro-sphere can be obtained fromimages of the visible photoluminescence. The green emission is due tospontaneous emission from the up converted F_(9/2) level to the groundstate of Er³⁺ and traces the path taken by the 980 nm pump wave withinthe sphere 9. The image in FIG. 6 shows a ring encircling the equator ofthe sphere. This equatorial ring corresponds to resonant pumping of anear fundamental WG mode. For this taper-sphere combination, and withresonant pumping of an equatorial WG mode, the scattering loss of thetaper-sphere junction is less than 5% (as measured by the off-resonancetransmission), and roughly 85% of the pump power is absorbed by themicrosphere.

[0042] Lasing in the microsphere 9 is rather complex, owing to the largenumber of high-Q modes that are present in the sphere 9, the spatialselectivity of the pump 20, the loading of the sphere 9 as a result ofthe taper 12, the large spectral gain bandwidth, and the variations inthe emission and absorption cross sections versus wavelength in thephosphate materials. For this reason other resonator geometries such asdisks, rings or racetracks may be preferable to obtain a simplifiedresonator spectrum.

[0043] Depending on the gain region within the sphere, lasing occurredat wavelengths ranging from 1530 to 1560 nm in both multimode andsingle-mode fashion. By adjusting the taper 12 contact position on thesphere 9 and the pump 20 wavelength, it is possible to switch betweenmulti-mode and single-mode lasing action. Single-mode lasing wasobtained in this embodiment by tuning the pump wavelength to afundamental WG mode resonance that produced a narrow equatorial-ringgain region. A typical single-mode lasing spectrum (as collected by thetaper 12) for an equatorial-ring pump region is shown in FIG. 7. Toresolve the fine spectral features of the laser (different m modes) ahigh-finesse (˜10,000) scanning Fabry-Perot cavity with a spectralresolution of a few megahertz was used to obtain the spectra shown inthe inset of FIG. 8. The microsphere of this embodiment of the presentinvention will lase on a single m WG mode over the entire pump rangedepicted in FIG. 8.

[0044] This embodiment of the present invention was also self-pulsingunder the pump conditions identified herein, with a period of roughly 15ms and a pulse width of 500 ns. Instability in the output of thisembodiment can be linked to the large unpumped highly absorbing regionswithin the sphere 9 and the nonlinear dynamics associated withabsorption saturation. A plot of the laser power collected in the taper12 versus the total pump power absorbed and scattered by the presence ofthe sphere 9 (L_(out)−L_(in)) is shown in FIG. 8. The lasing thresholdfor this embodiment in this configuration is estimated at 60 μW, and thelaser 22 can reach an output power of 3 μW while remaining single mode.A collected power as high as 10 μW was obtained in a single line athigher pump power, although the laser 22 was multimode. Given that thisembodiment and configuration used the same taper 12 as was used tocouple in the 980-nm pump power in the earlier described embodiment, tocouple out the 1.5 μm laser power from the sphere 9, and since the taperwas designed to phase match at the 980 nm pump wavelength to reduce thelasing threshold, the laser emission of this embodiment is not optimallycollected by the taper 12. A dual-taper system, as is described earlierand in the Cai and Vahala reference above identified, could be employedto likely improve the differential output efficiency.

[0045] A further embodiment is the use of multiple resonators on asingle fiber waveguide 5. This ability to cascade a series of devices isillustrated in FIG. 9, where two phosphate glass microspheres 21, 23 arepositioned along a single fiber taper, one after the other. Themicro-cavity devices can be the same or different sizes, depending onwhat the use and purpose the cascading is intended to achieve. FIG. 9shows a taper with two different-sized microspheres 21, 23 attached. Thelaser shown in FIG. 9(a) has a wavelength of 1535 nm; the laser shown inFIG. 9(b) which has a second microsphere 23 placed in contact with thefiber taper, a second laser line at 1533 nm appears. Thus, utilizingmultiple resonators in a single fiber can be used to create a laserarray.

[0046] Each of the characteristics in the present invention are believedto be new and unique, and are not found in the prior art. While theimplementations described below are directed to embodiments of a laserwhich utilize a tapered fiber and a microsphere resonator, it will beunderstood by those skilled in the art that such configurations and/orcombinations are merely embodiment of the present inventions. Thus, noneof the embodiments are intended to be limitations on the scope of theinvention described herein and set forth in the claims below.

We claim:
 1. A micro-cavity laser comprising: a. A fiber waveguide, saidfiber waveguide having a tapered coupling region, said tapered couplingregion being positioned between said first end and said second end ofsaid fiber waveguide; b. A micro-cavity optical resonator, saidmicro-cavity optical resonator being arranged so as to provide opticalcoupling between said tapered coupling region of said fiber and saidmicro-cavity optical resonator, said micro-cavity optical resonatorhaving at least one optical resonance at a desired frequency output,said micro-cavity including an active medium capable of providingoptical gain upon pump excitation; and, c. At least one laser pump, theoutput of said laser pumps being optically connected to said first endof said fiber waveguide to couple optical pump power into said resonatorto excite at least one resonance to pump said active medium, and inducelasing action such that laser output power is coupled to said fiberwaveguide.
 2. A micro-cavity laser of claim 1 further including a secondfiber waveguide, said second fiber waveguide having a coupling regionbetween a first end and a second end of said second fiber waveguide,said second fiber waveguide being optically coupled to said micro-cavityoptical resonator.
 3. The micro-cavity laser of claim 2 furtherincluding a second set of at least one laser pumps, the output of atleast one of said second set of laser pumps is optically connected tosaid first end of second fiber waveguide and the output of said secondset of laser pumps excites at least one resonance in said micro-cavityoptical resonator and thereby pumps the active medium to induce lasingaction.
 4. The micro-cavity laser of claim 3 wherein the output of atleast one of said second set of laser pumps excites at least oneresonance in said micro-cavity optical resonator at a frequencydifferent from the resonance excited by the output of said laser pumpsoptically connected to said first fiber waveguide.
 5. The micro-cavitylaser of claim 2 wherein the said first fiber waveguide and said secondfiber waveguide are optically coupled to the same micro-cavityresonances.
 6. The micro-cavity laser of claim 5 wherein the said firstfiber waveguide preferentially couples laser pump power from said fiberwaveguide to the micro-cavity to attain lasing and said second fiberwaveguide preferentially couples laser output power from saidmicro-cavity to said second fiber waveguide.
 7. The micro-cavity laserof claim 1 wherein said micro-cavity optical resonator is one of amicrosphere, disk, ring, and racetrack.
 8. The micro-cavity laser ofclaim 1 wherein said micro-cavity is based on silica.
 9. Themicro-cavity laser of claim 8 wherein said silica-based micro-cavity isdoped with a rare earth element to provide an active medium.
 10. Themicro-cavity laser of claim 9 wherein said rare earth element is atleast one of erbium, ytterbium, praseodymium, neodymium, holmnium, andthulium.
 11. The micro-cavity laser of claim 1 wherein said micro-cavityoptical resonator is a semiconductor, said semiconductor being arrangedto be pumped electrically.
 12. The micro-cavity laser of claim 1 whereinthe material composition of said micro-cavity includes phosphate glass.13. The micro-cavity laser of claim 1 wherein said micro-cavity opticalresonator includes a plurality of micro-rings in a semiconductor. 14.The micro-cavity laser of claim 1 wherein said micro-cavity opticalresonator includes a plurality of micro-rings on an optical fiber. 15.The micro-cavity laser of claim 1 wherein said micro-cavity opticalresonator includes a plurality of photonic crystal cavities.
 16. Themicro-cavity laser of claim 1 wherein said micro-cavity opticalresonator is fabricated on a substrate.
 17. The micro-cavity laser ofclaim 1 wherein the micro-cavity optical resonator includes Bragggratings in the resonant mode path so as to provide increased spectralpurity of the lasing output.
 18. The micro-cavity laser of claim 17wherein the Bragg gratings in the resonant mode path are definedholographically.
 19. The micro-cavity laser of claim 1 wherein themicro-cavity optical resonator has at least one preferred outputfrequency, and further including a frequency selector in the mode pathof the micro-cavity optical resonator of at least one of the preferredoutput frequencies of the micro-cavity optical resonator.
 20. A systemfor producing laser emission in a desired wavelength band, the systemcomprising: a. A fiber waveguide, said waveguide having a first end anda second end and a tapered region therebetween, said tapered regionhaving a tapered diameter; b. A micro-cavity optical resonator, saidresonator having a mode path diameter, said micro-cavity resonator beingconstructed from a silica material doped with at least one dopant; c.Optical gratings, said optical gratings being position in the mode pathof at least one resonant frequency of said micro-cavity opticalresonator; d. An alignment structure, said alignment structure beingarranged to locate said microcavity optical resonator and said fiberwaveguide in proximity to one another so as to enable coupling betweensaid tapered region of said fiber waveguide and said micro-cavityoptical resonator; and, e. A laser pump, said laser pump being opticallyconnected to said first end of said fiber waveguide and being arrangedso as to launch one or more signals into said fiber waveguide, saidoptical pump signals having frequencies which excite resonances in themicro-cavity optical resonator to thereby pump at least one silicadopant to induce lasing emission within a desired output frequency bandof the system.
 21. The system of claim 20 wherein at least one of thelaser pump source signals is in the 980 nanometer emission band.
 22. Thesystem of claim 20 wherein the lasing emission is in the range of1300-1600 nanometers.
 23. The system of claim 22 wherein the output ofthe system is used in a telecommunications application.
 24. The systemof claim 20 wherein said taper section diameter and said mode pathdiameter are selected to provide optimal phased matching such that thecoupling efficiency for pump and laser emission is maximized.
 25. Thesystem of claim 20 wherein said fiber waveguide includes at least oneadditional optical resonator optically coupled thereto at the taperedsection. Said at least one additional optical resonator doped so as toenable operation as a laser and optically pumped by coupling to saidfiber waveguide.
 26. The system of claim 25 wherein said fiber waveguidehas at least one additional taper coupling sections therein and at leastone of said additional doped resonators is coupled to at least one ofsaid additional taper sections in said fiber waveguide.
 27. The systemof claim 20 wherein the system includes at least one additional fiberwaveguide, each said additional fiber waveguide is optically coupled tosaid micro-cavity resonator and arranged so as to permit additionallaser pumping of or laser emission coupling from said micro-cavityoptical resonator.
 28. The laser of claim 20 wherein said micro-cavityis a sphere, disk, ring or racetrack.
 29. The system of claim 20 whereinoptical gratings increase the spectral purity of the laser emission byforcing laser oscillation at a desired frequency.
 30. The system ofclaim 20 wherein said dopants include at least one of erbium, ytterbium,praseodymium, neodymium, holmnium, and thulium.
 31. The system of claim20 wherein said fiber waveguide is a panda fiber.
 32. A micro-cavitylaser comprising: a. A first fiber waveguide, said first fiber waveguidehaving an evanescent coupling region, said evanescent coupling regionbeing positioned between a first end and a second end of said fiberwaveguide; b. A micro-cavity optical resonator, said micro-cavityoptical resonator being positioned in proximity to said coupling regionof said first fiber so as to evanescently couple said fiber couplingregion and said micro-cavity optical resonator, said micro-cavityoptical resonator having at least one optical resonance at a desiredfrequency output, said micro-cavity optical resonator comprising anactive medium capable of providing optical gain when excited; and c. Alaser pump, said laser pump being optically connected to said first endof said fiber waveguide for the purpose of exciting said gain medium.33. The laser of claim 32 wherein said micro-cavity resonator isfabricated on a chip or substrate.
 34. The laser of claim 33 whereinsaid fiber waveguide is an etched fiber.
 35. The system of claim 33wherein said fiber waveguide is a D-fiber.
 36. The system of claim 33wherein said fiber waveguide includes polished fiber half-blocks. 37.The system of claim 33 wherein said fiber waveguide is a panda fiber.38. The system of claim 33 wherein said waveguide coupling section isphased matched to said resonator such that the pump coupling and laseremission collection efficiency are maximized.
 39. The micro-cavity laserof claim 33 wherein the micro-cavity optical resonator has at least onepreferred output frequency, and further including frequency modifiergratings, said gratings being disposed in the mode path of themicro-cavity optical resonator of at least one of the preferred outputfrequencies of the micro-cavity optical resonator.
 40. The micro-cavitylaser of claim 33 wherein said micro-cavity optical resonator includesat least one of a micro-disk, ring and racetrack.
 41. A micro-cavitylaser comprising: a. A fiber waveguide, said waveguide having a firstend and a second end and a tapered region therebetween, said taperedregion having a tapered diameter; b. A highly doped Erbium:Ytterbiumphosphate silica micro-sphere, said microsphere being arranged so as toenable weak optical coupling between said microsphere and said taperedregion of said fiber waveguide; and c. A laser pump signal, said laserpump signal being transmitted in said fiber waveguide through saidtapered region, said laser signal including a frequency which excites aresonance in said silica microsphere and pumps the erbium gain medium toinduce laser emission.
 42. A micro-cavity laser system comprising: a. Afiber waveguide, said fiber waveguide having at least one taperedcoupling region, said tapered coupling regions being located betweensaid first end and said second end of said fiber waveguide; b. Aplurality of micro-cavity optical resonators including a firstmicro-cavity resonator, each said micro-cavity optical resonator beingarranged in proximity to at least one of said tapered coupling regionsso as to provide optical coupling between said micro-cavity opticalresonator and said fiber waveguide through at least one said taperedcoupling region of said fiber waveguide, at least one of saidmicro-cavity optical resonators having at least one optical resonance ata desired frequency output, said first micro-cavity optical resonatorincluding an active medium associated therewith capable of providingoptical gain upon pump excitation; and c. At least one laser pump, theoutput of said laser pump being optically connected to said first end ofsaid fiber waveguide to couple optical pump power into at least saidfirst micro-cavity resonator to excite said active medium associatedwith said first micro-cavity optical resonator and induce lasing actionsuch that laser output power is coupled to said fiber waveguide.
 43. Themicro-cavity laser system of claim 42 wherein the laser system includesat least a second micro-cavity optical resonator, said at least a secondmicro-cavity resonator including an active medium, said active mediumassociated with said at least a second micro-cavity optical resonatorproviding optical gain upon pump excitation at a frequency differentthan in said first micro-cavity optical resonator.
 44. The micro-cavitylaser system of claim 42 wherein at least one of said micro-cavityoptical resonators is one of a microsphere, disk, ring, and racetrack.45. The micro-cavity laser system of claim 42 wherein said micro-cavityis based on silica.
 46. The micro-cavity laser system of claim 45wherein said silica-based micro-cavity is doped with a rare earthelement to provide an active medium.
 47. The micro-cavity laser systemof claim 46 wherein said rare earth element is at least one of erbium,ytterbium, praseodymium, neodymium, holmnium, and thulium.
 48. Themicro-cavity laser system of claim 42 wherein said plurality ofmicro-cavity optical resonators are semiconductor based, saidsemiconductor being arranged to be pumped electrically.
 49. Themicro-cavity laser system of claim 42 wherein the material compositionof at least one micro-cavity resonator includes phosphate glass.
 50. Themicro-cavity laser of claim 42 wherein said plurality of micro-cavityoptical resonators are fabricated on a substrate.
 51. The micro-cavitylaser system of claim 42 wherein said plurality of micro-cavity opticalresonator includes a plurality of micro-rings on an optical fiber. 52.The micro-cavity laser system of claim 42 wherein said plurality ofmicro-cavity optical resonator includes a plurality of photonic crystalcavities.
 53. The micro-cavity laser system of claim 42 wherein saidplurality of micro-cavity optical resonator is fabricated on a substratematerial which is a semiconductor.
 54. A method of creating a lasersignal of a desired frequency, the steps comprising: Launching at leastone signal into a fiber waveguide, said waveguide having a taperedcoupling region, said tapered coupling region being optically coupled toa micro-cavity resonator, said micro-cavity resonator containing a gainmedium and being resonant and critically coupled to the signal so as topermit excitation of the gain medium and lasing in a desired emissionband.
 55. A method of obtaining a laser signal within a desiredfrequency range, the steps comprising: Receiving a laser signal in awaveguide, said waveguide being optically connected to a fiberwaveguide, said fiber waveguide having a fiber tapered coupling regiontherein, said tapered coupling region being optically coupled to amicro-cavity resonator, said micro-cavity resonator having a resonanceat the desired output frequency, and said micro-cavity resonatorcontaining a gain medium capable of amplification at the desired outputfrequency and excitation from the said laser signal.
 56. A method offabricating a phosphorus glass microsphere for use in a micro-cavityresonator, the steps comprising: Melting a small piece of phosphorusglass material in a crucible, Stabilizing the temperature of said moltenphosphorus glass, Placing the tip of a silica fiber taper into themolten phosphorus glass, Extracting the silica fiber so that a smallphosphate taper is formed on the end of the silica fiber taper; Meltingthe end of the phosphate taper until a spheroid forms under surfacetension, Quickly cooling the phosphate sphere in a manner which avoidscrystallization of the phosphate in the spheroid to an extent whichwould interfere with the refractive properties of the spheroid as amicro-cavity optical resonator.
 57. The method of producing amicrosphere of claim 54 wherein said phosphorus glass material is dopedwith a rare earth element.
 58. The method of claim 55 wherein saiddopant includes Erbium.
 59. The method of claim 55 wherein said dopantincludes Ytterbium.